1

DETAILS OF SCEEW ENGINES.

299

on a portion formed with the link, and thecounterbalance obviates the weight of the

Fig. 155.

MESSES. BENNIE’S EXPANSION LINK.

valve and details affecting the motion. Thequadrant and hand lever are shown in section,to depict the mode of setting the connectingrod, when the engine is in motion, if requisite.Obviously, the main advantage with thevalve in question is, that the pressure ofthe steam cannot affect the surfaces in work-ing contact, and thus an equilibrium actionresults.

The tubular valve does not always receiveFig. 156.

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messp.s. matjdsla's’s expansion valve and casin

& sliding - motion, as the example by Messrs.Maudslay illustrates, as represented byPig. 156 . It is preferred by that firm to causethe valve to revolve within a barrel having

openings or ports of less width than those inthe valve, by which difference a variable cut offis effected. The motion is derived from theengine shaft, and transmitted to the valve byspur, mitre, and bevel gearing. One of thespur wheels, and the entire set of mitregearing, is illustrated in plan by Fig. 157—page 300. The position of the gearing in eleva-tion will be better understood by referring toPlate 19. Alluding again to Fig. 157, it willbe seen that the counter shaft has fitted on it,between the bearings, a clutch coupling, withdiscs on each side of the clutch. One of the discs,when in gear with the fixed pin, holds the valvestationary ; and the other, when coupled withthat opposite, shifts the valve. As the latterattainment causes the grade or cut off, it willbe requisite to further explain the same. Onthe counter shaft, between the bearing and thecoupling, is a grooved barrel, encompassed bya loose ring or boss fitted with a pin, whichpasses through the slot in the barrel and fitsin a straight groove in the shaft. Behind themitre gearing is seen a hand wheel and screw,which latter is connected to the boss on thebarrel. Obviously, then, on motion being im-parted to the hand wheel, the boss will advanceand recede, and the barrel will be turned incorresponding directions. It is almost needlessto add that the pitch of the curve of the groovein the barrel is the main consideration in pro-portion to the laps of the valve.

Messrs. Ravenhill also adopt mitre gearingfor expansion gear; but the valve, instead ofbeing tubular, is square, with curved flanges—the width being proportioned to the stroke ofthe piston, speed of the valve, and grades ofexpansion. The casing encloses the valve, and